The oak's standing in northern European culture is best explained by what it did rather than by what it meant, and the practical account is more impressive than the symbolic one.
Acorns fed pigs. The right to run swine in oak woods, pannage, was a valuable and legally defined property interest recorded in Domesday Book, where woodland is measured not by area but by the number of pigs it would support. Acorns were also eaten by people in hard years across Europe and as a staple in parts of the Mediterranean and, more extensively, in Native California, where several peoples ground and leached acorn meal as a principal food. The leaching is essential: acorns are high in tannins and are unpalatable and mildly harmful without it.
Timber built ships and roofs. Oak is strong, durable, resists rot when wet, and can be cleft along the grain into curved pieces that follow the shape of a hull, which is exactly what a wooden warship needs. The naval demand was enormous and shaped forestry policy in England, Spain and France for centuries.
Tannin from oak bark tanned leather, which was a major industry, and oak galls, growths induced by gall wasps, supplied the iron gall ink in which most European manuscripts and records from the medieval period to the nineteenth century were written. Nearly every surviving document of the period is written in a solution made from a tree parasite.
Culpeper used the bark, leaf and acorn as astringents, which is straightforwardly what a high-tannin material does.